Switched mode power supplies (SMPS) are becoming increasingly common as power supplies for a great variety of applications. For example, SMPS may be used as power supplies for driving LEDs, which may be used to replace incandescent lamps for illumination purposes. However, many other applications for switching power converters exist as practically any electric and electronic device which requires a DC power supply voltage (or current) can be connected to the power grid using SMPS.
Switching power converter may be operated in various modes. For example, switching converters may be operated, inter alia, with a fixed switching frequency and a variable on-time of the switch and with a fixed on-time of the switch and variable frequency. Regardless of whether a switching power converter operates with a fixed or a variable frequency, switching power converters may operate in continuous current mode (CCM) or discontinuous current mode (DCM). As different modes of operation (DCM with fixed on-time, DCM with fixed frequency, CCM with fixed frequency, etc.) usually require different concepts of controlling (regulating) the output voltage or the output current, some switching power converters are designed to operate only in a single mode (e.g., CCM, fixed frequency). However, switching power converters are often required to be able to provide a specific constant DC output voltage (or current) for a great range of AC input voltages (e.g., from 85 to 270 volts). In this case, the switching power converter is usually designed to handle both modes of operation, DCM and CCM, and a mode-switch from CCM to DCM occurs when the AC input voltage exceeds a defined threshold voltage, which is pre-set by circuit design.
To accomplish the control task mentioned above the input current (which is switched on and off by a power semiconductor switch) of the power switching converter is usually measured, e.g., using a measurement resistor that provides a voltage drop proportional to the current passing through it. The measured input current is usually compared to a reference value and a switch-off of the power semiconductor switch is triggered when the input current exceeds a threshold defined by this reference value. However, the power semiconductor does not switch-off the input current (sometimes also referred to as primary current) immediately with zero delay. Not only the power semiconductor switch exhibits an inherent switch-off delay. Furthermore, the comparator circuit (which compares the measured input current with the mentioned threshold) and other circuit components included in the control circuit (which controls the switching operation of the power converter) cause additional delays. As a result an over-shot of the input current (primary current) occurs, which may lead to excessive power dissipation in the semiconductor switch.
To avoid the mentioned excessive power dissipation, the effect of the mentioned delays should be eliminated (or at least partly compensated for). However, in known solutions this delay time compensation is either designed for DCM or for DCM. Thus, it would be useful to improve the control circuit of a SMPS such that excessive power dissipation is avoided independent form the mode of operation (DCM, CCM) of the switching power converter.